Partitioning of species-forming and impurity cations among growth pyramids of pinacoid and prism faces in crystals of beryl, cordierite, and beryllian indialite

Abstract

The face effect was observed in all individual compounds with beryl structure having grown at a rate of 1–8 unit cells a second, with spontaneous crystallization in the system beryl (or cordierite)–Mg,Ca/Cl,F–impurity, where impurities are Ti, V, Cr and/or Fe oxides. By “face effect” is meant a difference in chemical compositions and symmetry of growth pyramids of crystallographically nonequivalent crystal faces. In the studied compounds of beryl structural type the simple crystal forms, prism and pinacoid, are characterized by hexagonal (pseudohexagonal in cordierite) and rhombic symmetry of face patterns. Degrees and characters of distinction of growth pyramids of prism and pinacoid faces of beryls, beryllian indialites, cordierites and their varieties vary in a wide range depending on the accumulation of species-forming and impurity elements. Most likely, this effect appears when crystallographically nonequivalent faces entrap chemically different “building units” (clusters spontaneously formed in a feeding medium and composed of fragments of beryl-type structure). It is supposed that the final isomorphous substitution of an impurity for the species-forming element occurs immediately on the face and/or attached unit. The character, direction, and degree of isomorphism depend on the initial composition of the medium, type of the main compound to be formed, and the presence of synchronous impurity phases. The same ratio of Si and Al concentrations in building units of the prism-pinacoid couple of cordierite with and without Ti and V suggests the persistent action of the patterns of these faces on the medium in the range of sampled concentrations of impurities and rates of growth. The contrasting face effect with respect to Si and Al in an indialite-cordierite individual is due to a drastic dissymmetry of the latter. The Ti- or V-related effects of cordierite faces are negligible. The study of beryllian indialite and its varieties showed that in some cases changes in Si-Al-Mg composition of units feeding faces with different crystallographic patterns are quite the same, with the considerable evolution of the total composition of the crystal-forming medium. As to the face effects of beryllian indialites (chiefly, with respect to Ti, V, Cr, and Fe impurities), they are owing to a considerable difference in degree of isomorphism with the participation of these elements in the building units that feed pinacoid and prism faces. Beryls demonstrate drastically contrasting face effects with respect to Ti (up to 0.20 f.u.) and in the presence of impurities, to Al (up to 0.21 f.u.); distinct effects were related to Cr (0.06 f.u.), in all varieties to Mg (up to 0.10 f.u.), and quite rarely, to Fe (0.01 f.u.), which is seen from comparison of the schemes of isomorphism implemented in prism and pinacoid. The effects under consideration and their evolution in the process of development of individuals are also clearly recognized from crystallochemical formulas of the building units feeding faces with different crystallographical patterns. By the example of beryl, it has been shown that the face effect inversion with respect to a component during the crystal growth indicates the inversion of the degree of isomorphous substitution of this element in the process of evolution of the individual (because of a drastic change in phase formation in the system) rather than the absence of relationship between face pattern and effect. Thus, in the system beryl–Mg,Ca/F,Cl–TiO 2 the inversion of Mg-related effect (resulting from inversion of degrees of substitution Al 3+ → Mg 2+) is linked to a drastic increase in rutile amount (TiO 2) crystallized together with beryl at a certain stage of solution cooling.